1. Field of the Invention
This invention relates to a mold used for pressing glass material into glass optical products, such as lenses and prisms, and a method of producing such a mold.
2. Related Background Art
A recent lens manufacturing technique, in which lenses can be formed by pressing glass material without requiring polishing, has eliminated the complicated steps in conventional lens manufacturing methods and, at the same time, made it possible to manufacture lenses more easily and economically. In addition to lenses, the techniques has come to be used for the production of other glass optical elements, such as prisms or the like.
The material of such a mold, used for pressing glass material into glass optical elements, is required to excel in hardness, heat resistance, mold release and mirror-finish characteristics, etc. Regarding metals or ceramics as the material for this type of a mold and molds coated with said materials, an number of proposals have been made in, for example, Japanese Patent Laid-Open Application Nos. 49-5112, 52-45613, and 60-246230. However, these molds and coating materials are liable to be subject to oxidation or to be fused to, or generate fogging on, the surface of the glass object to be molded. Thus, they are unsuitable for use as mold material and coating material for optical glasses.
In view of this, there has recently been proposed a mold for forming glass products which is coated with diamond which is resistant to oxidation, has a high hardness and is not susceptible to chemical reaction with glass as compared with the materials (Japanese Patent Laid-Open Application Nos. 61-183134, 61-242922 and 1-301864). Although the diamond layer has excellent properties suitable for the coating layer of a mold, in many cases it does not adequately adhere to the base material. As a result of the change from room temperature to that for glass molding, it often happens that the diamond layer is lifted or separated from the mold base material. In extreme cases, the diamond layer formed on the base material at a temperature from about 800.degree. C. to 1000.degree. C., may be lifted and separated from the mold solely by cooling it down to room temperature. This is assumed to be attributable to the coefficient of linear expansion of diamond, which is one digit smaller than that of the mold base materials generally in use. In this regard, the diamond layers formed by ion beam sputtering, as disclosed in Japanese Patent Laid-Open Application Nos. 61-183134 and 61-242922, are no exception to the problem. The higher the quality of the diamond, i.e., the less the amount of the amorphous carbon constituent thereof, the more liable to be separated from the base material.
It is rare for a mixed coating layer of diamond, graphite and amorphous carbon to be separated from the mold (Japanese Patent Laid-Open Application No. 1-301864) as a result of temperature change. However, in the case of a film with much amorphous carbon constituent, in particular, the amorphous carbon wears out as the molding is repeated. The result is that only the diamond crystals conspicuously remain, thus making it impossible to retain the required level of surface precision, i.e., a maximum surface roughness of 200 .ANG..